Carbon is always on the move. The carbon atoms in your body were, not long ago, in the air, and before that perhaps in a leaf, a fish or a seashell. The carbon cycle is the set of flows that moves carbon among the air, oceans, living things and rocks. Understanding it explains why carbon dioxide is rising, where it comes from, and why it lingers.

The fast cycle and the slow cycle

In the fast cycle, plants pull CO₂ from the air through photosynthesis, and animals, microbes and fires return it through respiration and burning. The ocean surface trades CO₂ with the air constantly. These flows are enormous, but for thousands of years before industry they were nearly balanced, so the amount in the air stayed close to 280 parts per million (ppm).

In the slow cycle, volcanoes release carbon from deep in the Earth, and the weathering of rocks slowly locks it away again, eventually in limestone on the seafloor. This cycle runs over hundreds of thousands to millions of years and acts as the planet's long-term thermostat. Fossil fuels belong to the slow cycle: coal, oil and gas are the remains of ancient life buried for millions of years. Burning them moves that carbon back into the fast cycle in a matter of decades.

The Keeling Curve

In 1958 a young chemist named Charles David Keeling began measuring CO₂ on the slopes of Mauna Loa in Hawaii, far from cities and forests. His first readings were about 315 ppm. Within a few years two patterns appeared. The level zigzags every year, falling in the northern summer as forests leaf out and rising in winter as leaves decay, like the planet breathing. And beneath the zigzag, the level climbs steadily. That record, still running, is called the Keeling Curve. CO₂ passed 400 ppm in 2015 and now stands at more than 420 ppm, about half again the pre-industrial level and higher than at any time in at least 800,000 years.

How we know the extra carbon is ours

Several independent lines of evidence point the same way.

• Bookkeeping: governments and companies track how much coal, oil and gas is sold and burned. Human activity now releases around 40 billion tonnes of CO₂ a year, more than enough to explain the rise.
• Chemical fingerprints: fossil carbon is millions of years old, so it contains no radioactive carbon-14, and plant-derived carbon is low in the heavier carbon-13. As fossil carbon builds up, the air's share of both isotopes has dropped exactly as expected.
• Missing oxygen: burning uses oxygen, and precise measurements show atmospheric oxygen falling slightly, in step with fuel use.
• The ocean is absorbing, not releasing: CO₂ in the surface ocean is rising too, so the ocean cannot be the source.

Where the carbon goes

Of the CO₂ we emit, a bit under half stays in the atmosphere. The ocean absorbs roughly a quarter and land plants and soils take up roughly another quarter or more. These natural sinks help, but they come at a price. When CO₂ dissolves in seawater it forms carbonic acid. The surface ocean has become about 30 percent more acidic since the Industrial Revolution, which makes it harder for corals, oysters and some plankton to build their shells and skeletons. This is called ocean acidification.

CO₂ is also stubborn. Much of a pulse of emissions is absorbed within a few centuries, but a substantial fraction stays in the air for thousands of years. Warming caused by CO₂ is, on human timescales, close to permanent.

The other gases

Methane, released by fossil fuel leaks, cattle, rice paddies, landfills and wetlands, has more than doubled since pre-industrial times. It traps far more heat per molecule than CO₂ but breaks down in about a decade, so cutting it slows warming quickly. Nitrous oxide, mostly from fertilizer, lasts more than a century. Water vapor is the most abundant greenhouse gas of all, but as Lesson 5 explains, it acts as a feedback rather than a driver.

Worked example: is it volcanoes?

A common claim says a single volcanic eruption releases more CO₂ than all of humanity. Test it with numbers.

• Volcanic total: geologists estimate that all of the world's volcanoes, on land and under the sea, release a few hundred million tonnes of CO₂ a year.
• Human total: roughly 40 billion tonnes a year, about a hundred times more.
• Check the record: the 1991 eruption of Mount Pinatubo, one of the largest of the century, left no visible jump in the Keeling Curve, while the steady climb continued.
• Check the fingerprints: volcanic carbon would not cause the drop in oxygen that burning produces.

The claim fails every test. Volcanoes matter over geological time, but today's rise is the signature of fossil fuels.